In semiconductor device manufacturing, projection optical systems are used to transfer integrated circuit (IC) patterns such as large scale integrated (LSI) circuit patterns from a reticle, also known as a mask, onto a wafer or semiconductor substrate upon which the semiconductor device is to be formed. Because of the difference in the relative sizes of the reticle and the resulting semiconductor device, also known as a die or a semiconductor chip, the projection optical system must be a reduction projection optical system.
Modern integrated circuits are becoming more integrated; that is, more and more functions are being integrated into circuits that are to be included in a single die. At the same time, however, there is a major effort not to allow the die to grow in size in order to maintain or improve the performance and speed of the semiconductor device being manufactured. In order to maintain the same or reduced die size, the reduction projection optical system must have a wider exposure area and a higher resolution.
The drive toward higher density circuitry in microelectronic devices has increased interest in a variety of high resolution lithographic techniques that produce finer resolution patterns at high production rates. The resolution of a lithographic lens system is a function of the exposure wavelength and the numerical aperture of the projection lens system. The resolution or minimum resolvable feature size is directly proportional to wavelength and inversely proportional to the numerical aperture, as follows (a lower value for resolution indicates better resolution):
resolution=k.lambda./NA, PA1 0.50&lt;T.sub.G5 /L.sub.G5 &lt;0.85, PA1 0.5&lt;T.sub.G1-G5 /L.sub.G1-G5 &lt;0.64, and PA1 0.50&lt;T.sub.G1-G5 /L&lt;0.70,
where k is a proportionality constant, .lambda. is the exposure wavelength and NA is the numerical aperture. One method to achieve better resolution of the optical system is to illuminate the reticle with shorter wavelength illumination and/or to use a projection lens system with a higher numerical aperture. In principle, the shorter the wavelength and/or the higher the numerical aperture of the projection lens system, the better the resolution. In the search for shorter wavelength illumination sources, there have been identified several excimer lasers that can be used as illumination sources for semiconductor photolithography, including the KrF excimer laser with a wavelength of 248 nanometers and the ArF excimer laser with a wavelength of 193 nanometers. These excimer lasers replace the traditional illumination sources that have been used for semiconductor manufacturing such as the deuterium lamp or the xenon-mercury arc lamp. The xenon-mercury arc lamp supplies the g-line that has a wavelength of 436 nanometers and the I-line that has a wavelength of 365 nanometers. These two ultraviolet lines have been the mainstay illumination used in semiconductor wafer manufacturing.
One of the advantages of using an excimer laser as an illumination source is that the excimer laser can produce an average power of several watts at a number of wavelengths. The high brightness of the excimer laser light source can either be used to obtain extremely fast exposures or a significantly smaller numerical aperture can be used in the projection lens system and still obtain reasonable exposure times. A smaller numerical aperture results in a larger depth of field that increases quadratically with the inverse of the numerical aperture. One advantage of a larger depth of field permits larger tolerances in wafer distortion and focus that leads to better lithographic patterns.
The excimer gas selected for use in the excimer laser may include only one gas, in which case the output is at the wavelength that is characteristic of the single excimer gas. The choice of which excimer gas to use depends on several factors, such as the characteristics of the photoresist being used in the semiconductor manufacturing process. For example, the KrF excimer gas produces an illumination output at 248 nanometers that is suitable for exposing photoresists such as a novolak resin sensitized with a diazo ketone.
Because an excimer laser has sufficient power at a single wavelength, another major advantage associated with using an excimer laser is that the aberration correction is simplified because the optics can be designed for a single wavelength. Because a single wavelength is used in a particular lens system means, for example, that chromatic aberration problems would be minimized.
As the integrated circuits (ICs) have become more sophisticated, the projection optical systems utilized to transfer the circuit patterns onto the semiconductor wafers or other receptive substrates (e.g. glass plates, etc.) have been required to achieve higher and higher performance levels. These higher performance levels include achieving higher resolution and the maintenance of high levels of aberration correction or the achieving of higher levels of aberration correction at large exposure field areas.
In order to obtain these higher performance levels in the deep ultra violet region of the spectrum, the projection optical lens systems have become very costly since increasing the numerical aperture and exposure field leads to larger diameter lenses and to an increase in volume of the materials required to produce or manufacture the lens elements of the projection lens systems. The high cost of the prior art projection lens systems with typical numerical apertures of 0.5-0.54 is partially attributed to the numerous lens elements and lens sizes necessary to achieve the required diffraction limited performance levels. The high cost has resulted from the fact that the lens elements are made of very expensive materials. In order to project light efficiently in the deep ultra violet region of the electromagnetic spectrum, it has been necessary to use expensive materials such as quartz for the lens elements.
One of the major problems with utilizing the shorter wavelengths provided by the excimer laser illumination is the limited availability of suitable optical materials that can be used in the wavelength range of the excimer laser illumination. The principal reason the conventional optical materials are unsuitable is that the transmission factor of most of the conventional optical materials is too limited to be used at these short wavelengths. Currently, ultraviolet grade fused silica (SiO.sub.2), generally referred to as quartz, and ultraviolet grade fluorite (CaF.sub.2) with extremely high homogeneity in refractive index are among the few optical materials that are considered feasible for use in short wavelength systems.
Accordingly, there is a need for projection lens systems that use fewer lens elements, small lens diameters and compact or short reticle-to-wafer conjugate distances and that are capable of maintaining or increasing the required high level of performance.